The invention relates to a dioptric lens system having an object side lens, an intermediate lens and an image side lens.
Such a dioptric lens system is known from document DE 42 34 721 A1. This known lens system as shown in FIG. 29 comprises an object side (front) lens 1, an intermediate lens 2 and an image side lens 3. A rear diaphragm 4 is provided behind the image side lens 3. The object side surface 5 of lens 1 is aspherical. As clearly shown in FIG. 29 the object side lens 1 is close to the intermediate lens 2 and a larger spacing is provided between the image side lens 3 and both front lenses.
Owing to the rear diaphragm 4 which functions as aperture diaphragm the known lens system can advantageously be used in the infrared region for cooled plane detectors in a Dewar having an inner cold shield. Further, the known lens system has a relatively short overall length. In FIG. 29 the rear principal plane defining the focal length of the lens system is indicated by the straight line A. As can be easily seen from FIG. 29 the overall length extends from the image plane by a factor of nearly 2 of the focal length.
Thus, the lens assembly is still relatively long. Moreover, all three object side surfaces of the three lenses of the lens system are aspherical. This results in increased costs in producing the lens system.
Owing to the fact that the exit pupil is formed by a rear diaphragm the entrance pupil is located as a virtual image of the exit pupil behind the lens system in light direction. This results in oversized diameters of the lenses and in particular of the front lenses which are positioned far in front of the entrance pupil.
The U.S. Pat. No. 2 234 707 discloses a photographic lens system having an object side lens with a positive refractive power and an intermediate lens with a positive refractive power, whereby the object side lens and the intermediate lens are combined in a screw mounting. An image side lens is formed as a dispersing lens. A rear diaphragm forming the exit pupil of the lens system is an entrance opening into the photographic apparatus.